This guide explains it (see page 2) - in a nutshell, fill your sprayer with water and spray a hard surface like concrete (on the driveway is fine) and work out how much water you use to spray 10m2. Simply then multiply it by 10 to get 100m2.

Proforce Duke label rates are as follows;
For Wintergrass or Rye, label rates are 150g/HA = 1.5g/100m2
For broadleaf control it is 100g/HA = 1g/100m2
For onion weed it is 25g/HA = 1/4g/100m2

The rates seem miniscule but it is because the product is specialised and design for broadacre application.